der Unfug
nonsense / mischief
Pointless, foolish, or disruptive behavior that is usually harmless but can cause trouble; nonsense or pranks.
Hört auf mit dem Unfug und macht eure Hausaufgaben!
Stop this nonsense and do your homework!

nonsense / mischief
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nonsense / mischief
Pointless, foolish, or disruptive behavior that is usually harmless but can cause trouble; nonsense or pranks.
Hört auf mit dem Unfug und macht eure Hausaufgaben!
Stop this nonsense and do your homework!

The word consists of the prefix 'Un-' and the old word 'Fug'. The prefix 'Un-' expresses something negative or wrong here. 'Fug' used to mean 'that which is fitting or right'. So 'Unfug' is literally something inappropriate or wrong.
Was ihr da macht, ist wirklich Unfug.
What you're doing there is really nonsense.
'Unfug' is an uncountable noun and is almost always used without an article and only in the singular. You cannot say 'ein Unfug' or 'viele Unfüge'.
Die Kinder haben im Garten nur Unfug im Kopf.
The children have nothing but mischief on their minds in the garden.
Common verbs used with 'Unfug' are 'Unfug machen' (to make mischief / talk nonsense) or 'Unfug treiben' (to get up to mischief). A typical command to stop is: 'Hör auf mit dem Unfug!' (Stop this nonsense!)
Die Schüler trieben während der Pause Unfug im Klassenzimmer.
The students got up to mischief in the classroom during the break.
nonsense
'der Quatsch' and 'der Unfug' both have to do with nonsense. 'Quatsch' can be a false or stupid statement, but also silly behavior. 'Unfug' especially means pointless or disruptive behavior, often pranks or things that can cause trouble. That is why the words can be confused when children, friends, or colleagues are being silly. When it is about a statement, 'Quatsch' is usually more natural. When someone does something foolish or disruptive, 'Unfug' fits better.

Im Zug machen die Kinder Unfug und verstecken die Fahrkarten ihrer Eltern.
On the train, the children are making mischief and hiding their parents' tickets.
Mein Nachbar hat erzählt, dass morgen alle Busse ausfallen, aber das ist Quatsch.
My neighbor said that all buses are canceled tomorrow, but that's nonsense.